The instrumental profile given by the authors of the Liège Atlas is refined by a trial-and-error procedure, in which the instrumental profile is convoluted with the profile of the telluric A band observed at very high resolution with the McMath spectrometer at Kitt Peak. A good approximation to the effective instrumental profile at λ7600 Å results from convoluting the profile (scaled according to wavelength) quoted by the authors with a gaussian function of half-width 30 mÅ, and has a half-width of 63 mÅ. Some effects of the instrumental profile are discussed.